1. Field of the Invention
The present invention relates to the field of networks and to methods and apparatus for congestion control.
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2. Background Art
Computer networks allow communication between one or more computers. Networks include local area networks (LANs), wide area networks (WANs), the Internet, wireless networks, mixed device networks, and others. One limitation to the efficient use of networks is network congestion, when the number of message sources and destinations, and the amount of message traffic, is greater than the network can handle efficiently. In the prior art, such congestion problems are handled by implementing congestion control.
Congestion control is a distributed algorithm to share network resources among competing users. It is used in situations where the availability of resources and the set of competing users vary over time unpredictably, yet efficient sharing is desired. These constraints, unpredictable supply and demand and efficient operation, have been solved in the prior art by using feedback control. Feedback control, also referred to as “closed loop” control, involves the use of some metric to determine dynamic and typically real-time adjustment of a system to provide optimum results. Such systems are distinguished from so called “open loop” control systems where there is no feedback (for example, cooking a turkey without using a meat thermometer is open loop and using a meat thermometer is a closed loop feedback system).
In this prior art approach, traffic sources dynamically adapt their rates in response to congestion in their paths. An example of a network that uses feedback control as a congestion control is the Internet (using Transmission Control Protocol (TCP) in source and destination computers involved in data transfers). Note that although we discuss the Internet, the present application applies to other networks as well.
The congestion control algorithm in the current TCP, also known as “Reno”, was developed in 1988 and has gone through several changes since. Current research predicts that as bandwidth-delay product continues to grow, TCP Reno will eventually become a performance bottleneck. In other words, the very control system used to manage congestion will lead to inefficient use of the network even as the network (Internet) continues to offer higher bandwidth and performance. The following four difficulties contribute to the poor performance of TCP Reno in networks with large bandwidth-delay products.    1. At the packet level, linear increase by one packet per Round-Trip Time (RTT) is too slow, and multiplicative decrease per loss event is too drastic. Current schemes use this “speed up slowly/slow down quickly” approach to packet traffic control and it is not effective in high bandwidth systems.    2. At the flow level, maintaining large average congestion windows requires an extremely small equilibrium loss probability, and maintaining such a small loss probability is not practical in prior art systems.    3. At the packet level, oscillation is unavoidable because TCP uses a binary congestion signal (packet loss).    4. At the flow level, the dynamics are unstable, leading to severe oscillations that can only be reduced by the accurate estimation of packet loss probability and a stable design of the flow dynamics. Current systems do not allow for accurate enough estimation of packet loss.